Field of the Invention
The invention relates to a method and a device for signaling a transmission time and/or a system clock, especially in a digital television transmission system.
Discussion of the Background
In a digital television transmission system, for example, according to the US-American Advanced-Television-Systems-Committee (ATSC) standard or according to the Digital-Video-Broadcast (DVB) standard conventional in Europe, time information is transmitted between a header station associated with the broadcaster and the individual television receivers, in each case in order to reconstruct respectively synchronize the system clock used by the broadcaster and to determine the display time based upon this system clock of every individual image of the television transmission on the screen of the television receiver.
In the case of the Moving-Picture-Expert-Group (MPEG) coding—Moving Picture Expert Group coding—conventionally used in television transmission, the system clock used by the broadcaster is transmitted in the Program-Clock-Reference (PCR) field—Program Clock Reference Field—of an MPEG-coded transport data packet of the display time of an image on the screen of the television receiver in the Presentation-Time-Stamp (PTS) field (Presentation Time Stamp Field) of an MPEG-coded transport data packet, and the time of the decoding of an image in the television receiver is transmitted in the Decoding-Time-Stamp (DTS) field (Decoding Time Stamp field) of an MPEG-coded transport data packet from the header station of the broadcaster to the television receiver.
Additionally, the transmission times of the individual MPEG-coded transport data packets of the transport datastream transmitted from the header station to the individual television receivers are transmitted between the header station of the broadcaster and an output adapter disposed downstream of the header station respectively between the broadcaster's header station and the individual transmission stations of a common-wave network.
While the system clock and the times for the display of an image or output of a sound respectively for the decoding of an image or of a sound are already generated by the video respectively audio source disposed in the studio, which each pack the recorded video respectively audio data into transport data packets or control data of a transport datastream respectively elementary datastream associated with the recorded program, the individual transmission times of the individual transport data packets are only determined in the case of the generation of a transport datastream in the header station connected downstream of the studio.
The system clock is obtained in the video respectively audio source on the basis of accumulated clock pulses of a clock source associated with the video respectively audio source and transmitted as time information in given transport data packets of a transport datastream respectively in given control data of an elementary datastream to the header station. In the header station, the time information containing the system clock is packed into transport data packets of the transport datastream. Additionally, in the header station, the transmission times of the individual transport data packets are obtained on the basis of accumulated clock pulses of a clock source associated with the header station and also stored as time information in given transport data packets of the transport datastream.
The precision of the system clock stored in this manner in the time-information fields of the individual transport datastreams respectively elementary datastreams and of the individual stored transmission times is quite substantially dependent upon the accuracy of the clock sources used in each case. While the clock source of a video respectively audio source typically provides a good average clock accuracy, the clock source used in a process computer of the header station is unfortunately characterized by only a low clock precision in the short-time horizon. The clock precision of the clock source used in the header station in the long-time horizon is comparatively good because of a time synchronization of the clock source used in the header station with a reference clock source with a good clock precision, for example, with the reference clock of the Global-Positioning-System (GPS) or of the Network-Time-Protocol (NTP) server within a relatively large time raster.
From DE 10 2009 057 362 A1, a method for generating a transport datastream in a television transmission system is known, in which the clock source used in the studio provides a significantly poorer clock precision by comparison with the clock source used in the header station. In this context, the clock precision of the clock source associated with a header station, which typically provides a high clock precision only in the long-time horizon, is improved in its clock precision in the short-time horizon by means of interpolation of the clock of an NTP-server responsible for the long-time horizon. In spite of the interpolation, the clock source of the header station provides an undesired phase jitter, which leaves the clock precision of the clock source associated with the header station with a quality also unsatisfactory for the demands of contemporary television transmission systems in the short-time horizon.